44 PA THOGENIC BA CTERIA.
pendent upon their previous existence in the soil, its pul-
verization, and its distribution by currents of the atmo-
sphere. Koch has shown that the upper stratum of the
soil is exceedingly rich in bacteria, but that their num-
bers decrease as the soil is penetrated, until below a0
depth of one meter there are very few. Remembering
that bacteria can live only upon organic matter, this is
readily understandable. Most of the organic matter is
upon the surface of the soil. Where, as in the case of
porous soil or the presence of cesspools and dung-heaps,
the decomposing materials are allowed to penetrate to a
considerable depth, the bacteria may occur much farther
from the surface, yet they are rarely found at any great
depth, because the majority of the known species require
oxygen.
The water of stagnant pools always teems with bacte-
ria, but that of deep wells rarely contains many unless
it is polluted from the surface of the earth.
Being generally present in the soil, which the feet of
men and animals grind to powder, the bacteria, together
with the pulverized earth, are blown from place to place
into every nook and cranny, until it is impossible to es-
cape them. It has been suggested by Soyka that the
currents of air passing over the surface of liquids might
take up bacteria, but, although he seemed to show it ex-
perimentally, it is not generally believed. Where bac-
teria are growing in colonies they seem to remain un-
disturbed by currents of air unless the surface becomes
roughened or broken.
Most of the bacteria which are carried about by the air
are what are called saprophytes, and are perfectly harm-
less to the human being; but not all belong to this class,
nor will they do so while tuberculous patients are al-
lowed to expectorate upon the sidewalks, and typhoid
patients' wash to dry upon the clothes-line, and their
dejecta to be spread upon the ground.
The growth of bacteria is profoundly influenced by
environment, so that a consideration of the conditions